To slim down a backlight used in a liquid crystal display device or the like, it is required to omit an additional sheet, such as a light diffusion sheet, and also reduce the thickness of a light guide plate itself.
However, when the thickness of the light guide plate is reduced, the light guide plate becomes slim like a sheet, and tends to be warped. And, when the light guide plate tends to be warped, assembling a backlight is difficult, and light can be leaked from a warped portion of the light guide plate. Therefore, as a method of preventing warping of the slimmed light guide plate, there is a method of affixing the light guide plate to the back surface of a liquid crystal display panel with an adhesive or the like without interposing an air layer between the light guide plate and the liquid crystal display panel.
An example of a liquid crystal display device with a light guide plate bonded to the back surface of a liquid crystal display panel is described in Patent Document 1 (Japanese Unexamined Patent Application Publication No. 5-88174), first comparative example (FIG. 3 of Patent Document 1). In this liquid crystal display device 11, as depicted in FIG. 1, an acrylic plate (having a refractive index of 1.49) with both surfaces being smooth is used as a light guide plate 12, and a connection layer 13 (a two-part-curable silicone having a refractive index of 1.51) having a refractive index higher than that of the light guide plate 12 is used to laminate the light guide plate 12 to the back surface of a scattering-type liquid crystal display panel 14 without interposing an air layer therebetween. Also, at a position facing each of both end faces of the light guide plate 12, a light source 15 formed of a cold-cathode tube is located.
In this liquid crystal display device 11, as depicted in FIG. 2(a), a light beam L emitted from the light source 15 and then entering the inside of the light guide plate 12 passes from the light guide plate 12 to a connection layer 13, and further enters the liquid crystal display panel 14 for scattering at a pixel in a scattered state (a whitish state), thereby being outputted forward to cause the pixel to emit light.
However, in this liquid crystal display device, because the refractive index of the connection layer 13 is higher than the refractive index of the light guide plate 12, as a light beam L indicated by a dotted line in FIG. 2(a), total reflection of light does not occur at an interface between the connection layer 13 and the light guide plate 12. Therefore, the light beam L entering the light guide plate 12 is not guided inside the light guide plate 12, and ends up in being emitted from the liquid crystal display panel 14 near the light source 15. As a result, as depicted in a luminance distribution of FIG. 2(b), a portion near any of the light sources 15 has a high luminance of light emission and is bright, but a portion away from any of the light sources 15 (that is, a center portion between the light sources 15) has a low luminance of light emission and is dark.
To solve unevenness in luminance of light emission as described above, in a first embodiment (FIG. 1 of Patent Document 1) described in Patent Document 1, as depicted in FIG. 3, a thin film 16 having a refractive index lower than that of the light guide plate 12 is partially formed on the surface of the light guide plate 12. Also, the area rate of the thin film 16 at a portion near any of the light sources 15 is set large, and the area rate of the thin film 16 at a portion away from any of the light sources 15 is set small. Furthermore, the light guide plate 12 having the thin film 16 formed thereon is laminated to the back surface of the liquid crystal display panel 14 via the connection layer 13 having a refractive index higher than that of the light guide plate 12. Here, the light guide plate 12 is formed of an acrylic plate having a refractive index of 1.49, a two-part-curable silicone having a refractive index of 1.41 is used as the thin film 16, and a two-part-curable silicone having a refractive index of 1.51 is used as the connection layer 13.
In the first embodiment of Patent Document 1, because the thin film 16 is formed on the surface of the light guide plate 12, light inside the light guide plate 12 is totally reflected off the interface between the light guide plate 12 and the thin film 16, thereby being guided inside the light guide plate 12. Moreover, because the area rate of the thin film 16 is large at a portion near any of the light sources 15, the ratio of the light passing through gaps between each thin film 16 to be emitted from the liquid crystal display panel 14 is small. Because the area rate of the thin film 16 is small at a portion away from any of the light sources 15 with a small amount of light reached, the ratio of light passing through gaps between each thin film 16 to be emitted from the liquid crystal display panel 14 is large. As a result, the luminance of light emission can be made uniform over the entire display surface of the liquid crystal display device.
Furthermore, in a second embodiment (FIG. 2 of Patent Document 1) described in Patent Document 1, as depicted in FIG. 4, asperities 17 in a prism shape are partially formed on the surface of the light guide plate 12, and the degree of surface roughness of the asperities 17 is made low at a portion near any of the light sources 15, and the degree of surface roughness of the asperities 17 is made high at a portion away from any of the light sources 15. Also, the light guide plate 12 having the asperities 17 formed thereon is laminated to the back surface of the liquid crystal display panel 14 via the connection layer 13 having a refractive index lower than that of the light guide plate 12. Here, the light guide plate 12 is formed of an acrylic plate having a refractive index of 1.49, and a two-part-curable silicone having a refractive index of 1.41 is used as the connection layer 13.
In the second embodiment of Patent Document 1, because the refractive index of the connection layer 13 is lower than the refractive index of the light guide plate 12, in a smooth region on the surface of the light guide plate 12, light inside the light guide plate 12 is trapped inside the light guide plate 12 due to total reflection, and is guided inside the light guide plate 12. On the other hand, light entering the asperities 17 is scattered by the asperities 17, thereby passing though the inside of the connection layer 13 and further being scattered at a pixel of the liquid crystal display panel 14 in a scattered state for light emission. Moreover, because the degree of surface roughness of the asperities 17 is low at a portion near any of the light sources 15, the ratio of light scattered at the asperities 17 and emitted from the liquid crystal display panel 14 is small. Because the degree of surface roughness of the asperities 17 is high at a portion away from any of the light sources 15 with a small amount of light reached, the ratio of light scattered at the asperities 17 and emitted from the liquid crystal display panel 14 is large. As a result, the luminance of light emission can be made uniform over the entire display surface of the liquid crystal display device.
In the first embodiment of Patent Document 1, the directivity characteristic of light inside a plane perpendicular to the light guide plate 12 is depicted in FIG. 5(a). The spread (directivity characteristic) of light immediately before entering the light guide plate 12 is ±90°, but because the refractive index of the light guide plate 12 is ng=1.49, the spread of light immediately after entering the light guide plate 12 is represented as±arcsin(1/1.49)=±42.2°.On the other hand, a critical angle of total reflection at the interface between the light guide plate 12 and the thin film 16 is represented as±arcsin(1.41/1.49)=71.1°.This critical angle of 71.1° corresponds to 18.9° when measured from a horizontal direction.
Thus, among light beams entering the inside of the light guide plate 12 with the spread of ±42.2°, light beams within a range of 18.9° to 42.2° and light beams within a range of −18.9° to −42.2° when measured from a horizontal direction (light beams within a range with broken lines in FIG. 5(b)) pass through the thin film 16 without being reflected off the thin film 16 when entering the thin film 16. In this manner, light beams within a range with broken lines in FIG. 5(b) pass through the thin film 16 near the light sources 15 and are not guided inside the light guide plate 12. Therefore, light with a sufficient amount cannot be guided away from the light sources 15, and the luminance of light emission cannot be sufficiently made uniform.
Also in the second embodiment of Cited Reference 1, the refractive index of the connection layer 13 is 1.41, which is equal to that of the thin film 16 in the first embodiment. Therefore, among light beams entering the inside of the light guide plate 12 with the spread of ±42.2°, light beams within a range of 18.9° to 42.2° and light beams within a range of −18.9° to −42.2° when measured from a horizontal direction pass through the connection layer 13 without being reflected off a smooth region of the light guide plate 12. In this manner, also in the second embodiment, the light beams within the range with the broken lines in FIG. 5(b) pass through the connection layer 13 near the light sources 15 and are not guided inside the light guide plate 12. Therefore, light with a sufficient amount cannot be guided away from the light sources 15, and the luminance of light emission cannot be sufficiently made uniform.
Note that in the specification, to represent the directivity characteristic and the directivity spread of light, a conventional notation may be used. For example, instead of representing the spread of light as described above, that is, −42.2° to +42.2° (that is, −42.2≦χ≦+42.2 where the spread of light is taken as χ), the spread of light may be represented simply as ±42.2°.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 5-88174